The present invention relates to a controller of an engine that can use a blended fuel with alcohol blended therein.
Although generally, gasoline is used as a fuel in an engine of an automobile or other vehicle, a blended fuel with alcohol blended therein can also be used. For example, there is a vehicle (FFV: Flexible Fuel Vehicle) equipped with an engine that can use a blended fuel in which alcohol is blended at any proportion (0% to 100%).
An alcohol concentration of a blended fuel supplied to such an engine for FFV is not necessarily constantly fixed. The alcohol concentration of the blended fuel supplied to the engine changes according to the alcohol concentration of the blended fuel supplied as fuel and the fuel supplying amount. Also, alcohol is higher in octane number than gasoline, and the octane number of the blended fuel changes according to the alcohol concentration. An air-fuel ratio, ignition timing, etc., must thus be adjusted appropriately according to the alcohol concentration in the blended fuel. For example, there is a configuration where, when, during low speed rotation of the engine, the alcohol concentration of the blended fuel (fuel used) is high, the ignition timing is corrected to a spark advance side, and when, during high speed rotation of the engine, the alcohol concentration of the blended fuel is high, the ignition timing is corrected to a spark retardation side (see, for example, JP-A-5-33748).
As described above, when, during high speed rotation engine, the alcohol concentration of the blended fuel is high, it may be possible to prevent knocking by correcting the ignition timing to the spark retardation side. However, this causes an engine output to decrease. Also, when the ignition timing is corrected to the spark retardation side, an exhaust temperature rises and this may cause heat damage of an exhaust system.
Because the alcohol used as fuel is approximately ⅔ in energy density in comparison to gasoline, to obtain an equivalence ratio substantially equal to that in a case of using gasoline when a blended fuel of comparatively high alcohol concentration (for example, a blended fuel having alcohol blended at a proportion of 85%) is used, a fuel injection amount from a fuel injection valve must be increased by up to approximately 1.5 times. Thus, in a case where a fuel injection valve used in a gasoline engine is applied as it is to an engine using a blended fuel, when a large amount of fuel is to be injected, for example, in a high-speed, high-load state, a demanded injection amount may exceeds a maximum injection amount (limiting injection amount) of the fuel injection valve and a desired equivalence ratio may not be obtained. Though obviously this problem does not arise if a special fuel injection valve for blended fuel is used, since this leads to an increase in cost, it is desirable to use a fuel injection valve used in a gasoline engine.